This invention relates generally to automatic density control for a copying machine.
In general, the capability of a copying machine changes with time. For instance, the density of a copy varies as either the supply of toner becomes inadequate or the developing bias voltage changes. Accordingly, it is necessary to automatically control these two factors both before or between copying operations. In order to do so, the copying machine should preferably be operated on a trial basis; however, during this operation, a large amount of toner is consumed. In order to eliminate this difficulty, in the prior art density controllers produce toner which is allowed to stick to only a small area (typically 35 cm.sup.2) of the surface of the photosensitive drum for the purposes of density testing.
In the technology of this invention, a developing bias voltage is controlled by feeding back the density thus detected, since a bias voltage responds to a control signal quickly. However, since it is difficult to completely control the density merely by controlling the developing bias voltage, the rotation of a toner supply motor is also controlled in the present invention.
An automatic control of this type has been known in the art; however, a number of factors such as the light intensity of a lamp, charge current, developing bias voltage and the density of toner must also be controlled in such prior art system. As such, these prior art density control systems are unduly complicated and highly unreliable. The above-described method of utilizing a small part of the surface of the drum has also been proposed in the art; however, in the conventional method, two areas are selected in such a manner that one of the two areas is high in toner density, while the other is low in toner density. The densities of the two areas are subjected to comparison to determine a suitable density. This method is also intricate to control and is not practical.